The invention relates to an arrangement of busbars for conducting the direct electric current from the ends of the cathode bars of a transversely disposed electrolytic cell, in particular a cell for producing aluminum, to the anode beam of the following cell whereby some of the busbars are positioned under the cell.
In order to produce aluminum by the electrolysis of aluminum oxide, the aluminum oxide is dissolved in a fluoride melt which is made up for the most part of cryolite. The cathodically deposited aluminum collects under the fluoride melt on the carbon floor of the cell where the surface of the liquid aluminum forms the cathode. Dipping into the melt from above are anodes which in conventional processes are made of amorphous carbon. The anodes are secured to an overhead anode beam. As a result of the electrolytic decomposition of the aluminum oxide, oxygen is formed at the carbon anodes and combines with the carbon to form CO.sub.2 and CO. The electrolytic process takes place in general at a temperature of about 940.degree.-970.degree. C. During the process, the electrolyte becomes depleted in aluminum oxide. When this substance reaches a lower concentration of 1-2 wt.%, an anode effect occurs which causes an increase in voltage from, for example, 4-5 V to 30 V and more. At this time the solidified crust of electrolyte on the surface must be broken open and the concentration of aluminum oxide increased by adding fresh aluminum oxide (alumina).
Embedded in the carbon floor of the cell are cathode bars, the ends of which project through the sides of the tank of the cell. These iron bars collect the electrolyzing current which flows via the busbars outside the cell through the risers, the anode beam and the anode rods to the carbon anodes of the next cell. An energy loss of the order of up to 1 kWh/kg of aluminum produced is experienced in passing current from the cathode bars to the anodes of the next cell as a result of ohmic resistance. Repeated attempts have been made to optimize the arrangement of the busbars with respect to ohmic resistance. In doing so, however, the vertical components of induced magnetic fields must be taken into account; together with the horizontal components of current density, these produce a significant magnetic field in the liquid metal produced in the reduction process.
The passage of current from cell to cell in an aluminum smelter with transversely arranged reduction cells is as follows: The direct electric current is collected by the cathode bars embedded in the carbon floor of the cell and leaves, with respect to the general direction of current flow, via the upstream and downstream ends. The iron cathode bars are connected via flexible strips to aluminum busbars. The busbars which, if desired, may be in the form of collector bars, conduct the direct current to the vicinity of the next cell where the current is led via other flexible strips and risers to the anode beam supporting the anodes. The risers are, depending on the type of cell, connected electrically to the end and/or a longitudinal face of the anode beam.
These characteristic arrangements for conducting the electrolyzing current in aluminum smelters suffer, however, from difficulties both of electrical and magnetic nature; efforts to overcome these have been reported in many publications.
In the British patent GB No. 1 032 810 an invention which concerns the hooding of the cell discloses that the busbars can be arranged below the electrolytic cell. The electric current is fed from the long side of the cell, symmetrically into the anode beam of the next cell. According to FIG. 2 of the patent conductors 135 are made to pass symmetrically under the cell with respect to the transverse direction of the cell.
According to the U.S. Pat. No. 3,415,724 an arrangement of busbars is aimed at, by means of which the magnetic effects are not increased when the current is increased. To this end, a part of the current emerging from the cathode bar ends at the upstream end--but less than half of the current, is led under the cell. The rest of the current leaving the cathode bar ends at the upstream end is led, in a concentrated manner around the end of the cell. According to FIG. 3 of the patent the conductors which carry the current under the cell are positioned in the middle of the cell and are shown as collector bars. The feeding of current into the anode beam of the next cell takes place, with respect to the transverse axis of the cell, symmetrically at four places on the long sides of the anode beam.
The process disclosed in the German Auslegeschrift No. 26 13 867 shows an arrangement of busbars according to which a part of the current leaving the cathode bars in the upstream direction, is fed via two busbars in the middle of the cell, under the cell and into the side of the anode beam of the next cell. The rest of the current emerging upstream is carried around the cell and fed into the end face of the anode beam of the next cell (FIG. 3) of the patent. The current flowing out of the cathode bars at the downstream end is led to the other branch of the anode beam of the next cell and fed in at the side.
The arrangement shown in the German patent application No. 28 45 614 to compensate for harmful magnetic effects comprises three collector busbars running under the cell. The current is fed via risers into the side of the anode beam of the next cell. This manner of feeding is however asymmetric as a small amount of current is led around that short side of the cell which faces the magnetically dominating, neighboring row of cells. The publications representing the state of the art or the devices described in them, where some proportion of the busbars are positioned under the cells, have the disadvantage that the magnetic and electrical difficulties cannot be overcome in an optimal fashion.
It is therefore a principal object of the present invention to provide an arrangement of busbars for transversely disposed electrolytic cells, whereby negligible magnetic and electrical effects are produced at low investment costs and with good electrical efficiency.